Porous films with straight pores oriented normal to the plane of the films are a desirable geometry for a wide range of applications. Prior to this invention the state of the art for producing accessible vertical pores consisted of: optical lithography, e-beam serial lithography, ion track etching, anodic alumina, aligned diblock copolymers, and various surfactant or polymer template materials including magnetic field alignment of silica mesopores. Bicontinuous cubic pores do not have anisotropy between the in-plane and out of plane directions, but have homogenous porosity with a component that is normal to the substrate. Optical lithography is limited to features above 30 nm in pitch for EUV techniques [1,2]. E-beam lithography is a serial method. The rest are parallel processes like that described here, but each has drawbacks.
Track etching uses ion beams of heavy, high energy ions to irradiate a sample. The samples are dielectrics and semi-insulating materials like kapton with no polycrystallinity; the technique is most often applied onit to polymer films. The preferential etching of degraded material in the path of the ion allows cylindrical column pores to be etched out of the target. Single pores can be 6 nm or larger and have aspect ratios in excess of 1000. While it has no order, the density of pores is controlled by exposure time and feature density can reach 1014/cm2 [3,4]. Throughput with this technique is limited by the collimation of the ion beam because of its reliance on ion trajectory.
Aluminum can be anodically etched to produce vertical alumina pores. The process produces a disordered pore structure with diameters between 6 nm and 200 nm that is thermally stable to 800° C. As a film, this process is only able to produce pores in aluminum and only on conductive substrates [5].
Block copolymers are two or more polymers covalently bound at one point. They microphase segregate based upon the miscibility of the blocks. This can be used to produce a material with hexagonal symmetry. In order for the cylinders to be vertically aligned there are several methods that have been applied. Heating above the glass transition in the presence of an electric field allows the system to reorganize locally about a preferred axis defined by the electric field. Specially treated surfaces that have equal contact angles to both volumes of a diblock copolymer allow orientation to be defined by the thickness of the film. When the thickness is n+ 1/2  the pore to pore distance, and the polymer is heated above the glass transition temperature, frustration of the interface leads to vertical orientation of the volumes [6]. This technique only works for small aspect ratios. Recently, addition of nanoparticles to a melt has been shown to equalize surface energy and produce high aspect ratio volumes. Polymers are limited to temperatures below their melting points which decrease with size, limiting the practical diameters. To get small diameter cylinders, it is necessary to use small polymers. Microphase segregation only produces ordered phases above a minimum product of the interaction energy and the polymerization number. This results in a practical lower limit on cylinder diameters to about 20 nm. Once the cylinder is oriented, differential etching or crosslinking is used to make a pore [7, 8, 9].
Templated inorganic systems rely on the interaction of a templating micelle (surfactant or polymer) and a polymerizable inorganic (FIG. 1). They self assemble into a matrix which is inherently ordered and can be made porous by removing the organic component. Bicontinuous cubic phases can be produced [10, 11, 12]. Once calcined, these have a 3D pore system that is open to the top and the bottom of the film. There is no orientation control and the pores are only connected through small necks between the cubic sites. Generally, templated inorganic systems have high temperature stability and a wide variety of materials are available for the matrix, from conductors to insulators. A 2-D hexagonal system produces long cylindrical pores. Unfortunately, on a homogenous substrate they form pores only parallel to the substrate. To change this orientation, it is necessary to use magnetic fields in the range 10 T or greater [13,14]. In-plane orientation can also be controlled through the substrate, but until now it has only been possible to control orientation parallel to the substrate [15].
Ordered inorganic pore systems can be produced by self assembly using an organic polymer or surfactant template and inorganic matrix precursors as in FIG. 1. Unfortunately, hexagonal pores produced by self assembly on homogenous substrates produce horizontal pores as in FIG. 2.